As one effective way for enabling miniaturization of electronic devices, multifunctional electronic components are used in electronic devices. Examples of the multifunctional electronic components include ceramic multilayer modules.
The ceramic multilayer module includes a laminate ceramic substrate. Wiring conductors, which are for performing the function of electrical connection or for configuring passive elements such as a capacitor and an inductor, are incorporated in the laminate ceramic substrate, and various electronic components are mounted on the laminate ceramic substrate.
In this way, the ceramic multilayer module can be multifunctionalized though it is small-sized, and this is used to make miniaturization of electronic devices possible.
Further, the demand of higher frequency for the electronic devices have increased in addition to the above miniaturization of electronic devices. In the context of such situations it is desired that the laminate ceramic substrate provided in ceramic multilayer modules used in a high-frequency domain be superior in high-frequency characteristics. More specifically, it is desired that an insulating ceramic sintered body used as an insulating ceramic layer to provide a laminated structure in the laminate ceramic substrate be superior in high-frequency characteristics.
Examples of insulating ceramic compositions for obtaining the insulating ceramic sintered body which can satisfy such demands include, for example, a composition described in Japanese Unexamined Patent Publication No. 2008-37739 (Patent Document 1).
Patent Document 1 discloses a glass ceramic composition including a first ceramic powder having forsterite as the main constituent, a second ceramic powder containing at least one selected from the group consisting of a ceramic powder having calcium titanate as the main constituent, a ceramic powder having strontium titanate as the main constituent and a ceramic powder having titanium oxide as the main constituent, and a borosilicate glass powder containing lithium in an amount of 3 to 15 weight % on a Li2O equivalent basis, magnesium in an amount of 20 to 50 weight % on a MgO equivalent basis, boron in an amount of 15 to 30 weight % on a B2O3 equivalent basis, silicon in an amount of 10 to 45 weight % on a SiO2 equivalent basis, zinc in an amount of 6 to 20 weight % on a ZnO equivalent basis, and aluminum in an amount of 0 to 15 weight % on an Al2O3 equivalent basis.
In this glass ceramic composition, the above borosilicate glass powder accounts for 3 weight %, and at least one additive constituent selected from the group consisting of calcium oxide, barium oxide, and strontium oxide is added to the borosilicate glass. When the content of the additive constituent is represented by the percentage of the borosilicate glass powder, the lower limit of the content of the additive constituent is 2 weight % in terms of the sum of the calcium oxide content on a CaO equivalent basis, the barium oxide content on a BaO equivalent basis and the strontium oxide content on a SrO equivalent basis, and the upper limit of the content of the additive constituent is 15 weight % on the CaO equivalent basis in the case of calcium oxide, 25 weight % on the BaO equivalent basis in the case of barium oxide, and 25 weight % on the SrO equivalent basis in the case of strontium oxide.
In accordance with the glass ceramic composition described in Patent Document 1, it is possible to fire a glass ceramic composition at a temperature of 1000° C. or lower, and a glass ceramic sintered body, which is obtained by this firing, has excellent chemical stability, a relatively low relative permittivity and a higher Q value, and the temperature coefficient (τf) of a resonance frequency is stable. Therefore, if the ceramic substrate is formed by use of the glass ceramic sintered body, copper or silver can be used as the main constituent of the wiring conductors provided there, and a ceramic substrate suitable for high-frequency applications can be formed.
However, the glass ceramic composition described in Patent Document 1 has a problem in that the Q value of a sintered body thereof is relatively low and the glass ceramic composition is inferior in chemical resistance, such as plating solution resistance.
On the other hand, when a capacitor is configured in the laminate ceramic substrate, for example, the ceramic sintered body ceramic layer located in association with the capacitor desirably has a high dielectric constant.
Examples of the high dielectric ceramic composition for obtaining a ceramic sintered body capable of satisfying such demands include a ceramic composition described in International Publication WO 2008/018408 (Patent Document 2).
Patent Document 2 discloses a glass ceramic composition formed by including a SrZrO3 based ceramic and a Li2O—MgO—ZnO—B2O3—SiO2 based glass, wherein the Li2O—MgO—ZnO—B2O3—SiO2 based glass accounts for 1 to 12 weight % of the total, where the content of Li2O is 3.5 to 15 weight %, the content of MgO is 20 to 50 weight %, the content of BaO is 0 to 25 weight %, the content of CaO is 0 to 10 weight %, the content of SrO is 0 to 25 weight %, the content of B2O3 is 16 to 29 weight %, the content of SiO2 is 11 to 35 weight %, the content of ZnO is 5 to 20 weight %, and the content of Al2O3 is 0 to 15 weight %, and wherein the glass ceramic composition further contains a SrTiO3 based ceramic in an amount of 0 to 6 weight % of the total.
In accordance with the glass ceramic composition described in Patent Document 2, it is possible to sufficiently achieve sintering at low temperature while maintaining characteristics of the SrZrO3 based ceramic even if an additive amount of the above glass is small since the Li2O—MgO—ZnO—B2O3—SiO2 based glass exhibits extremely good wettability on a SrZrO3 based ceramic. Accordingly, the high relative permittivity of the SrZrO3 based ceramic can be maintained in accordance with a glass ceramic sintered body obtained by firing this glass ceramic composition. Further, this glass ceramic composition deposits crystal phases of Mg3B2O6 and Li2MgSi2O8. Since these crystal phases exhibit the high Q value, the Q value of a sintered body thereof can be enhanced.
A laminate ceramic substrate having a combined structure including a low dielectric constant layer and a high dielectric constant layer laminated together has been proposed. When such a laminate ceramic substrate is manufactured, it is conceivable that the low dielectric constant layer is formed of a sintered body of the glass ceramic composition described in Patent Document 1 and the high dielectric constant layer is formed of the a sintered body of the glass ceramic composition described in Patent Document 2 from the point of view of the dielectric constant.
When such an laminate ceramic substrate having a combined structure is manufactured, it is necessary to co-fire the low dielectric constant layer and the high dielectric constant layer. In this case, it is preferred that the difference in thermal expansion coefficient between the low dielectric constant layer and the high dielectric constant layer is as small as possible. However, there is a relatively large gap between the thermal expansion coefficient of the sintered body of the glass ceramic composition described in Patent Document 1 and the thermal expansion coefficient of the sintered body of the glass ceramic composition described in Patent Document 2, and there is a possibility that defects such as peeling, cracks and pores may be developed in the laminate ceramic substrate.    Patent Document 1: Japanese Unexamined Patent Publication No. 2008-37739    Patent Document 2: International Publication WO 2008/018408